Замените <br> пробелом в выводе BeautifulSoap

Как насчет использования .get_text() с параметром-разделителем?

from bs4 import BeautifulSoup

page = '''<h1 class = "para-title">A quick brown fox jumps over<br>the lazy dog
<span>some stuff here</span></h1>'''


soup = BeautifulSoup(page, 'html.parser')
title_box = soup.find('h1', attrs = {'class': 'para-title'})
title = title_box.get_text(separator = " ").strip()
print (title)   

Выход:

print (title)
A quick brown fox jumps over the lazy dog
 some stuff here

Используйте функцию str.replace :
print title.replace("<br>", " ")

Использование replace() в html перед синтаксическим анализом:

from bs4 import BeautifulSoup

html = '''<h1 class = "para-title">A quick brown fox jumps over<br>the lazy dog
<span>some stuff here</span></h1>'''

html = html.replace("<br>", " ")
soup = BeautifulSoup(html, 'html.parser')
title_box = soup.find('h1', attrs = {'class': 'para-title'})
title = title_box.get_text().strip()
print (title)

ВЫХОД:

A quick brown fox jumps over the lazy dog
some stuff here

РЕДАКТИРОВАТЬ:

Для части OP, упомянутой в комментариях ниже;

html = '''<div class = "description">Planet Nine was initially proposed to explain the clustering of orbits
Of Planet Nine's other effects, one was unexpected, the perpendicular orbits, and the other two were found after further analysis. Although other mechanisms have been offered for many of these peculiarities, the gravitational influence of Planet Nine is the only one that explains all four.
</div>'''

from bs4 import BeautifulSoup

html = html.replace("\n", ". ")
soup = BeautifulSoup(html, 'html.parser')
div_box = soup.find('div', attrs = {'class': 'description'})
divText= div_box.get_text().strip()
print (divText)

ВЫХОД:

Planet Nine was initially proposed to explain the clustering of orbits. Of Planet Nine's other effects, one was unexpected, the perpendicular orbits, and the other two were found after further analysis. Although other mechanisms have been offered for many of these peculiarities, the gravitational influence of Planet Nine is the only one that explains all four..